Edible dispersions are well known in the art and include for example products such as spreads like margarine which are water-in-oil (W/O) emulsions, wherein an aqueous phase is dispersed in a fat continuous phase.
Generally food products like for example margarines and similar edible fat continuous spreads are prepared according to known processes that encompass the following steps:    1. Mixing of the liquid oil, the structuring fat and if present the aqueous phase at a temperature at which the structuring fat is definitely liquid;    2. cooling of the mixture under high shear to induce crystallization of the structuring fat to create an emulsion;    3. formation of a fat crystal network to stabilize the resulting emulsion and give the product some degree of firmness;    4. modification of the crystal network to produce the desired firmness, confer plasticity and reduce the water droplet size.
The structuring fat, also called solid fat or hardstock fat, serves to structure the fat phase (being the case in for example a shortening as well as in a water in oil emulsion) and helps to stabilize the aqueous phase, if present, by forming a fat crystal network.
These steps are usually conducted in a process that involves apparatus that allow heating, cooling and mechanical working of the ingredients, such as the churn process or the votator process. The churn process and the votator process are described in the Ullmans Encyclopedia, Fifth Edition, Volume A 16, pages 156-158. Typical examples of such apparatus used in the votator process are the scraped-surface heat exchanger and the pin-stirrer. The role of the pin-stirrers is the working of the product typically during which time the crystallization process further progresses to completion. In low-fat spreads often the pin-stirrer is also the location of phase inversion of the supercooled oil-in-water (O/W) premix into the final W/O emulsion.
A disadvantage of these processes is that the complete composition (including the liquid oil, structuring fat and if present the aqueous phase) is subjected to a heating step and a cooling step, which requires a lot of energy. Another disadvantage of the known processes is that the choice of fats that can practically be used as structuring agent is rather limited.
Alternative processes have been disclosed wherein the structuring fat is added as fat powder (i.e. crystallized fat) thereby eliminating the need to heat the whole composition to above the melting temperature of the structuring fat. An example of such a disclosure can be found in WO2010/069751.
In such an alternative process, since crystallization is carried out separately, the primary objective of the mixing unit is to create a homogenous dispersion of water in a continuous oil phase, with fine droplet size and firm texture. Contrary to the votator process there is no requirement for a minimal residence time based on the time required to complete any crystallization. In fact all residence time beyond performing the mixing and emulsification will merely contribute to viscous heating. The role of the mixing unit in such an alternative process is thus quite different from that of the pin-stirrer and surface-scraped heat exchanger in the traditional votator process. The primary role of the mixer in the alternative process is to be efficient in providing both high dispersive and distributive mixing. In contrast, in a pin-stirrer, high-shear zones for droplet break-up are only present close to the tips of the rotor pins, so only in a small part of the total volume. Therefore, apparatus such as the scraped surface heat exchanger and the pin-stirrer may not be considered an optimal choice as mixing device in a process using fat powder. There is a need for a mixer which provides efficient dispersive and distributive mixing to mechanically work the water phase, oil phase and the fat powder into an edible emulsion, preferably with a short residence of the mixture and a small working volume.
EP 1331988 B1 discloses a fluid dynamic mixer which may be used in fluid to solid mixing applications and may achieve a degree of particle size reduction. In particular, there is disclosed a dynamic mixer comprising two elements which are rotatable relative to each other about a predetermined axis and between which is defined a flow path extending between an inlet for material to be mixed and an outlet, wherein the flow path is defined between surfaces of the elements each of which surfaces defines a series of annular projections centered on the predetermined axis, the surfaces are positioned such that projections defined by one element extend towards spaces between projections defined by the other element, cavities are formed in each surface to define flow passages bridging the projections, cavities formed in one surface being offset in the axial direction relative to cavities in the other surface, and cavities in one surface overlapping in the axial direction with cavities in the other surface such that material moving between the surfaces from the inlet to the outlet is transferred between overlapping cavities. The cavities are characterized by having curved bases.
We have found that when a mixing apparatus, as disclosed in EP1331988 B1, is used in a process to provide dispersions made with fat powder, dispersions of poor quality and/or unpredictable quality may result. In particular, a recurring quality deficit of the spreads made using said mixing apparatus was observed: a large distribution of droplet size, leading to inhomogeneous spreads.
We have further found that said mixing apparatus has a high energy dissipation (a.k.a. energy consumption). Energy consumption leads to increased operational costs and undesired heating of the mixture. Undesired heating of the mixture above a critical temperature may reduce the quality and stability of the resulting edible dispersion.
It is an object of the present invention to provide a mixing apparatus which achieves good distributive and dispersive mixing without leading to undesired heating of the mixture.
It is a further object of the present invention to provide a mixing apparatus capable of being used in a process to prepare edible emulsions and do so in a wide window of operation, in particular a wide range of flow-rates.
It is a further object of the invention to provide a process to prepare an edible dispersion, preferably a dispersion of an aqueous phase in a fat continuous phase, with improved properties such as a small distribution of the droplet size of the dispersed phase.